Abstract A very large proportion of persons living with HIV infection (PLH) in Russia are not presently in HIV medical care, and many do not adhere to ART regimens at levels needed to achieve full viral suppression. Because viral suppression greatly reduces the likelihood of HIV transmission, poor care engagement fuels Russia's HIV disease incidence at a population level. This application proposes to expand a longstanding, bilateral, and interdisciplinary collaboration between investigators at the Center for AIDS Intervention Research (CAIR) at the Medical College of Wisconsin, USA and St. Petersburg State University Medical Faculty to develop and test a novel intervention to reach social networks of out-of-care PLH in the community and then harness peer social support to improve network members' HIV care engagement and adherence. In a 6-month qualitative phase, in-depth interviews will be conducted with 30 out-of-care or ART nonadherent HIV+ drug users, men who have sex with men, and heterosexual men and women to examine relationships between substance use and care nonengagement, and to elicit feedback and recommendations about the planned intervention. The study's main trial will evaluate the effectiveness of a social network intervention designed to improve care engagement and ART adherence. Our prior research in Russia has established that PLH in the community are often linked in their friendship groups with other HIV+ persons and that these networks can be recruited into research. The intervention will recruit 48 out-of-care or ART nonadherent HIV+ seeds from community settings in St. Petersburg. Seeds will invite their HIV+ friends, who will in turn invite their own HIV+ friends into the study, creating a sample of 48 PLH networks (expected n=288, 6 members/network x 48 networks). Following baseline assessment of care engagement, ART adherence, treatment attitudes, psychosocial distress, substance use, and CD4+ and viral load, 24 networks (n=144 participants) will be randomized to an intervention condition and 24 networks (n=144) to the comparison condition. All members of each intervention condition network will together attend a 4-session intervention to strengthen attitudes, intentions, and skills for entering, remaining, and adhering to HIV medical care. Because participants will attend sessions with other PLH who are their own friends in day-to-day life, the intervention will build and increase mutual social support within each network for HIV care and adherence. Peer champions identified in each intervention network will attend 3 additional sessions in which they are guided to reinforce and help to sustain friends' medical care engagement. Intervention outcomes will be determined by baseline to 6- and 12- month followup change on primary measures of participant attendance at HIV medical care visits, adherence to ART regimens, and viral load as well as secondary measures of alcohol use, drug use, sexual risk behavior, treatment attitudes, and psychosocial distress. This research will evaluate a new strategy to reach and deliver intervention to improve HIV medical care engagement among out-of-care PLH in the community.